Work machine with engine control device

ABSTRACT

A work machine with an engine control device is provided, which is capable of ensuring fine controllability and providing further improved fine controllability with a simple construction. To this end, the work machine includes: an engine control device for controlling the output of an engine in accordance with each of a plurality of operation modes which are set according to the contents of operations; and operation mode selector switches for selecting any one of the plurality of operation modes. If the operation mode selector switches select, from the plurality of operation modes, an operation mode for setting a set revolution speed for the engine to a relatively low value, the engine control device performs isochronous control for maintaining the revolution speed of the engine to a constant value irrespective of load fluctuations.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP2004/005175 filed Apr. 9, 2004.

TECHNICAL FIELD

The present invention relates to a work machine with an engine controldevice.

BACKGROUND ART

Hydraulic excavators, for example, are conventionally used for a widevariety of operations such as excavation, craning and leveling andtherefore are required to have the capability of performing effectiveexcavation, while keeping fine controllability needed for craning andleveling operations.

One hydraulic excavator which meets the above requirement is set out in,e.g., Japanese Patent Publication No. 3316057. The hydraulic excavatorproposed in this publication includes hydraulic actuators activated bypressure oil from a variable displacement hydraulic pump driven by anengine; an operating speed detecting means for detecting the change rateof operation amount of a control lever for operating a hydraulicactuator; and an engine revolution speed controlling means forcontrolling engine revolution speed. If the change rate of operatingamount detected by the operating speed detecting means is lower than aspecified value, the revolution speed of the engine is kept to a presetspeed by the engine revolution speed controlling means. Specifically,during the operation in which the change rate of operating amount of thecontrol lever is small such as craning and leveling,increasing/decreasing of engine revolution speed is inhibited so as notto affect the fine control. On the other hand, during the operation inwhich the change rate of operating amount detected by the operatingspeed detecting means is higher than the specified value, the enginerevolution speed controlling means increases the revolution speed of theengine from the preset speed according to the load imposed on thehydraulic actuator. That is, since the change rate of operating amountof the control lever during excavation is higher than those of craningand leveling operations, the revolution speed of the engine is increasedaccording to work load, thereby effectively performing excavation.

The hydraulic excavator disclosed in the above publication, however,cannot avoid complication of the control system, because it is designedto control the revolution speed of the engine by the engine revolutionspeed controlling means based on the change rate of operating amount ofthe control lever detected by the operating speed detecting means. Inaddition, since the threshold for determining whether the change rate ofoperating amount is high or low is set based on the operational feelingof the operator, the recall factor of the operational effect is likelyto vary under the influence of the physical condition of the operator orthe individual difference between the operators when a plurality ofoperators use the work machine in cooperation.

The present invention is directed to overcoming the foregoing problemand a primary object of the invention is therefore to provide a workmachine with an engine control device which is capable of ensuringreliable fine controllability and providing further improved finecontrollability with a relatively simple construction.

DISCLOSURE OF THE INVENTION

In accomplishing the above and other objects, there has been provided,in accordance with the present invention, a work machine with an enginecontrol device, the machine comprising:

hydraulic actuators activated by pressure oil from a hydraulic pumpdriven by an engine;

an implement driven by the activation of the hydraulic actuators;

an engine control device for controlling the output of the engine inaccordance with each of a plurality of operation modes which are setaccording to the contents of operations; and

operation mode selecting means for selecting any one of the plurality ofoperation modes,

wherein after a specified operation mode has been selected from theplurality of operation modes by the operation mode selecting means, theengine control device performs isochronous control for maintaining therevolution speed of the engine to a constant value irrespective of loadfluctuations.

In the invention, if a specified operation mode is selected by theoperation mode selecting means from the plurality of operation modeswhich are set according to the contents of operations, the isochronouscontrol is performed by the engine control device so that the revolutionspeed of the engine is maintained to a constant value irrespective ofload fluctuations. Therefore, even if a load fluctuation occurs, theoperational speed of the implement can be constantly maintained, therebyensuring good fine controllability. In addition, the invention has theadvantage that such an operational effect can be attained by arelatively simple control system which performs the isochronous controlonly when a specified operation mode is selected. Since this operationaleffect can be obtained without fail whenever the operation modeselecting means selects the specified operation mode, there is nolikelihood that the recall factor of the operational effect varies asseen in the conventional machine.

The invention is preferably designed such that the specified operationmode is a finely-controlled operation mode for allowing the implement tooperate at ultraslow speed which mode is among the plurality ofoperation modes, and if an operation mode for setting a set revolutionspeed for the engine to a value in the vicinity of a rated outputrevolution speed is selected from the plurality of operation modes bythe operation mode selecting means, the engine control device performsregulation control for increasing/decreasing the revolution speed of theengine according to load fluctuations. When the finely-controlledoperation mode suited for operation of the implement at ultraslow speedis selected, in more concrete words, the finely-controlled operationmode which is set so as to allow, for instance, a hydraulic excavator toproperly perform craning or leveling operation is selected, the enginecontrol device is allowed to perform the isochronous control, so thatthe implement can be easily operated at constant ultraslow speed througha relatively rough manipulation and as a result, further improved finecontrollability can be achieved. In addition, if the operation mode forsetting a set revolution speed for the engine to a value in the vicinityof the rated output revolution speed is selected from the plurality ofoperation modes by the operation mode selecting means, the regulationcontrol is performed by the engine control device, whereby the operatorcan grasp the degree of work load fluctuations based onincreases/decreases in the revolution speed of the engine. This enablesthe operator to make right manipulation in the course of operation sothat the operation can be smoothly carried out without troubles.

In the invention, it is preferable that the specified operation mode bethe finely-controlled operation mode for allowing the implement tooperate at ultraslow speed which mode is among the plurality ofoperation modes and an operation mode for setting a set revolution speedfor the engine to a value in the vicinity of a rated output revolutionspeed which mode is among the plurality of operation modes. This notonly increases the fine controllability similarly to the above-describedarrangement but also allows the engine control device to perform theisochronous control when the operation mode for setting a set revolutionspeed for the engine to a value in the vicinity of the rated outputrevolution speed is selected, so that even if the work machine issuddenly brought into an unloaded condition during operation, therevolution speed of the engine will not be increased. In addition, theset revolution speed for the engine during unloaded operation itself canbe set low, so that vibration and noise can be reduced.

In the invention, it is preferable that the engine control deviceperform equi-horse-power control subsequently to the isochronouscontrol, if the value of the output torque of the engine which the loadrequires still increases after reaching a specified value when thefinely-controlled operation mode is selected. This enables it toincrease the output torque while restraining changes in the revolutionspeed of the engine caused by increases in the load. As a result,high-load operation can be smoothly performed without impairing the finecontrollability. During this time, the output of the engine issubstantially constant, so that no wasteful energy consumption occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a hydraulic excavator constructed according toa first embodiment of the invention.

FIG. 2 is a block diagram showing a schematic structure of anengine/hydraulic control system according to the first embodiment.

FIG. 3 is an engine output torque characteristic graph according to thefirst embodiment.

FIG. 4 is an engine output torque characteristic graph according to thesecond embodiment.

FIG. 5 is a block diagram showing a schematic structure of anengine/hydraulic control system according to a third embodiment.

FIG. 6 is an engine output torque characteristic graph according to thethird embodiment.

FIG. 7 is a block diagram showing a schematic structure of anengine/hydraulic control system according to a fourth embodiment.

FIG. 8 is an engine output torque characteristic graph according to thefourth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, a work machine with anengine control device will be described according to preferredembodiments of the invention. These embodiments are associated withcases where the invention is applied to a hydraulic excavator which isone kind of work machines.

FIG. 1 shows a side view of a hydraulic excavator according to a firstembodiment of the invention.

The hydraulic excavator 1 has lower machinery 2 designed to freelytravel, being driven by a hydraulic motor for traveling (not shown);upper machinery 4 mounted on the lower machinery 2 through a swivel 3which uses a hydraulic motor for turning (not shown) as a drivingsource; and an implement 6 attached to the upper machinery 4. Theimplement 6 is constituted by a boom 7, an arm 8 and a bucket 9 whichare respectively pivotally arranged in this order from a side of theupper machinery 4. The boom 7, the arm 8 and the bucket 9 are pivotallyoperated by expansion/contraction of a boom cylinder 10, an arm cylinder11 and a bucket cylinder 12, respectively. The upper machinery 4includes an operator's cab 5 which has, therein, an operating system(not shown) for operating the hydraulic actuators (i.e., the hydraulicmotor for traveling; hydraulic motor for turning; boom cylinder 10, armcylinder 11 and bucket cylinder 12) and a monitor panel 20 (see FIG. 2)which is composed of a monitor for displaying various indicators and anoperating unit having various switches. In this hydraulic excavator 1, asuspending hook (not shown) is attached to a pin 14 for coupling thebucket 9 to a bucket link 13 which constitutes a pivotal movementmechanism for the bucket 9, so that not only excavation and leveling,but also craning can be performed.

Next, reference is made to the block diagram of FIG. 2 to fully describean engine/hydraulic control system according to the first embodiment ofthe invention.

The engine/hydraulic control system 15 of the first embodiment includesa diesel engine 16; a variable displacement hydraulic pump 17 driven bythe engine 16; an engine control device 18 for controlling the output ofthe engine 16; a pump control device 19 for controlling the deliverycharacteristics of the hydraulic pump 17; and operation mode selectorswitches (operation mode selecting means) 24 (an active mode selectorswitch 21, excavation mode selector switch 22 and lifting mode selectorswitch 23) which are provided in the operating unit of the monitor panel20, for selecting an operation mode from a plurality of operation modes(described later) set according to the contents of operations.

The engine 16 is equipped with a fuel injection pump 25 for emitting ajet of fuel into the fuel chamber of the engine 16. An explanation ofthis fuel injection pump 25 in conjunction with the drawings will beomitted. This pump 25 includes (i) a force-feeding mechanism composed ofa plunger for applying high pressure to the fuel to forcibly send to aninjection pipe and a cum shaft; (ii) a force-feeding amount adjustingmechanism which includes a control rack engaged with the plunger, foradjusting the feeding amount of the fuel pressure-fed by theforce-feeding mechanism, by changing the rack position of the controlrack.

The hydraulic pump 17 is connected to the hydraulic actuators through acontrol valve 26. In the control valve 26, switch-over of oil paths isdone by operation of each of operating levers which are disposed in theoperating system in correspondence with the hydraulic actuators. Theoperator operates each operating lever in a predetermined manner so thatits associated hydraulic actuator is supplied with pressure oil from thehydraulic pump 17 to allow the traveling motion of the lower machinery2, the turning motion of the upper machinery 4, or the flexing/hoistingmotion of the implement 6.

The engine control device 18 has (i) an electronic governor 27 forcontrolling the rack position of the control rack provided for theforce-feeding amount adjusting mechanism of the fuel injection pump 25;and (ii) an engine controller 28 for transmitting a governor drivesignal to the electronic governor 27. Input to the engine controller 28are engine revolution speed detection signal from a revolution sensor 29for detecting the revolution speed of the engine 16 and a throttlesignal from a throttle sensor 31 for detecting the operation amount of afuel dial 30.

The pump control device 19 is composed of a swash plate drive unit 32for inclining a swash plate provided for the hydraulic pump 17 and apump controller 33 for controlling the activation of the swash platedrive unit 32. Input to the pump controller 33 are a pump revolutionspeed detection signal from a revolution sensor 34 for detecting therevolution speed (=engine revolution speed) of the hydraulic pump 17 andan operation mode selection signal from the operation mode selectorswitches 24.

Signal transmission/reception is possible between the engine controller28 and the pump controller 33. The operation mode selection signal inputto the pump controller 33 from the monitor panel 20 is sent to theengine controller 28 as a mode command signal. The mode command signaltransmitted from the pump controller 33 is input to the enginecontroller 28. In the engine controller 28, the operation mode which hasbeen selected through the selecting operation of the operation modeselector switches 24 is identified based on the input mode commandsignal, and a predetermined governor drive signal is transmitted to theelectronic governor 27 so that the output characteristic of the engine16 becomes correspondent with the selected operation mode. Referencenumeral 35 designates a signal line for transmitting information aboutthe engine 16 to the monitor panel 20. The information about the engine16 transmitted through this signal line is displayed on the monitorprovided for the monitor panel 20. In the pump controller 33, based on atarget revolution speed signal which has been sent from the enginecontroller 28, indicating a target revolution speed for the engine 16set by the fuel dial 30 and based on a pump revolution speed detectionsignal indicative of the revolution speed of the pump detected by therevolution sensor 34, the discharge rate of the hydraulic pump 17 iscontrolled by the swash plate drive unit 32 such that the best matchingtorque at each output point of the engine 16 is taken in the hydraulicpump 17, and equi-horse-power control is performed in each operationmode in order to make a matching at a point where the fuel efficiency ofthe engine 16 is high (see the equi-horse-power curves designated by Paand Pb in FIG. 3).

The operation modes set in the first embodiment consist of three modes,i.e., active mode, excavation mode and lifting mode (finely-controlledoperation mode). Herein, the active mode is set in correspondence withoperation which requires speed and power. The excavation mode is forenabling ordinary excavating operation in an output region where thefuel efficiency of the engine 16 is good, whereas the lifting mode isset in correspondence with operation which requires fine controllabilitysuch as craning and leveling. In the first embodiment, the output of theengine 16 is controlled by the engine control device 18 in accordancewith each of the operation modes.

In the first embodiment, the following two types of control areperformed on the engine 16 by the engine control device 18.

One is called “regulation control (droop control)”. This regulationcontrol is such that after a target revolution speed for the engine 16is set by the fuel dial 30 during unloaded operation (idling) of theengine 16, the revolution speed of the engine 16 decreases as work loadincreases.

The other control is called “isochronous control”. In this isochronouscontrol, the revolution speed of the engine 16 is maintained to aconstant value, irrespective of the fluctuation of work load. Morespecifically, in the isochronous control, the engine controller 28determines a set revolution speed in response to a throttle signal sentfrom the throttle sensor 33 and a mode command signal sent from the pumpcontroller 33. Then, the engine controller 28 makes a comparison betweenthe set revolution speed and the actual revolution speed of the engine16, thereby determining a target rack position for the control rack ofthe fuel injection pump 25 and transmits a drive signal to theelectronic governor 27, for executing feedback control so as to make theactual rack position equal to the target rack position. Thus, the amountof fuel injection is controlled, thereby maintaining the revolutionspeed of the engine 16 to a constant value irrespective of thefluctuation of work load.

In the hydraulic shovel 1 of the first embodiment configured asdescribed above, if the operator turns ON the lifting mode selectorswitch 23 among the operation mode selector switches 24, the engineoutput torque characteristic line designated by EL_(A) in FIG. 3 is setand the engine control device 18 executes the isochronous control inaccordance with the engine revolution speed constant line designated byLa in FIG. 3. On the other hand, if the operator turns ON the activemode selector switch 21 among the operation mode selector switches 24,the engine output torque characteristic line designated by EL_(B) inFIG. 3 is set and the engine control device 18 executes the regulationcontrol in accordance with the inclining load line designated by Lb inFIG. 3. In the excavation mode selected by turning ON the excavationmode selector switch 22, an engine output torque characteristic, withwhich the set revolution speed is set to a value slightly lower thanthat of the active mode, is selected, and the control executed by theengine controller 18 is basically the same as the regulation control ofthe active mode. Therefore, the inclining load line associated with theexcavation mode of FIG. 3 is omitted from the drawing for convenience ofexplanation. In FIG. 3, the broken line designated by Lc is theinclining load line where the isochronous control is not performed butthe regulation control is performed in the lifting mode. In FIG. 3, theengine revolution speed in parentheses is the set revolution speed whenthe regulation control is performed in the lifting mode.

In the present embodiment, since the isochronous control is performed inthe lifting mode in which the set revolution speed for the engine 16 isrelatively low (set revolution speed: 1480 r.p.m.), the work machine 6can be easily operated at a constant ultraslow speed even by arelatively rough manipulation, so that the load does not sway whilecraning operation being carried out and the blade does not deviate froma course during excavation of a sloped land. In the active mode in whichthe set revolution speed for the engine 16 is set to a relatively highvalue equal to or in the vicinity of the rated output revolution speed(set revolution speed: 2050 r.p.m.), the regulation control is performedand therefore the operator can sense the degree of work load fluctuationbased on increases and decreases in the revolution speed of the engine.This permits the operator to make accurate manipulation in the course ofoperation so that the operation can be smoothly carried out withouttroubles. In addition, since the control system of the present inventionfor achieving the above effect can be relatively simply constructed anddoes not depend upon the operational feeling of the operator, there isno likelihood that the recall factor of the effect may fluctuate.

Next, a second embodiment of the invention will be described below. FIG.4 shows an engine output torque characteristic graph according to thesecond embodiment. The hardware configuration of the engine/hydrauliccontrol system of the second embodiment is basically the same as that ofthe first embodiment.

While the first embodiment has been discussed in terms of a case wherethe isochronous control is executed in the lifting mode whereas theregulation control is executed in the active mode, the second embodimentis designed such that, as shown in FIG. 4, the isochronous control isperformed in the lifting mode according to the engine revolution speedconstant line designated by Code La like the first embodiment and theisochronous control is also performed, in the active mode, according tothe engine revolution speed constant line designated by Code Ld.According to the second embodiment, improved fine controllability can beachieved like the first embodiment. In addition, even if the workmachine suddenly comes into an unloaded condition while performingoperation in the active mode, the revolution speed of the engine willnot increase, and furthermore, the set revolution speed for the engineduring unloaded driving itself can be set low, so that vibration andnoise can be reduced.

It should be noted that the first and second embodiments may employ thesame device as the engine control device 18A of the third and fourthembodiments (described later) in place of the engine control device 18.In the engine control device 18A, a common-rail fuel injection device40, the engine controller 28 and instruments including various sensorsconstitute an electronically controlled injection system, as describedlater.

Next, a third embodiment of the invention will be described below. FIG.5 is a block diagram showing a schematic structure of anengine/hydraulic control system according to the third embodiment. FIG.6 is an engine output torque characteristic graph according to the thirdembodiment. In the third embodiment, parts that are identical or similarto those of the foregoing embodiments are once again indicated with thesame reference numerals as in the foregoing embodiments. Although adetailed description of them is omitted herein, only parts inherent tothe third embodiment will be chiefly explained below.

The engine 16 is equipped with the accumulator (common-rail) fuelinjection device 40. The fuel injection device 40 itself is publiclyknown and therefore a detailed explanation of it with reference to thedrawings is omitted herein. The fuel injection device 40 is of a type inwhich fuel is accumulated in a common rail by a fuel force feed pump andthe fuel is injected from an injector by opening/closing of anelectromagnetic valve. The fuel injection device 40 is formed such thatfuel injection characteristics are determined based on a drive signalsent from the engine controller 28 to the electromagnetic valve so thatarbitrary injection characteristics over the range from the low-speedregion to high-speed region of the engine 16 can be obtained. In thethird embodiment, an electronically controlled injection system, whichis composed of the fuel injection device 40, the engine controller 28and instruments including various sensors, constitutes the enginecontrol device 18A. In such an electronically controlled injectionsystem, target injection characteristics are mapped by digital values,thereby obtaining the engine characteristics described later.

The engine controller 28 stores mapped engine output torquecharacteristics corresponding to the lifting mode and the active mode,respectively. Herein, in the third embodiment, an engine output torquecharacteristic line EL_(A)′ is set in connection with the lifting mode.The engine output torque characteristic line EL_(A)′ has an isochronouscontrol line La and is set such that the output torques of the middleand low speed regions are slightly lower than those of the engine outputtorque characteristic line EL_(A). In the third embodiment, an engineoutput torque characteristic line EL_(B) having the same regulation lineLb as that of the first embodiment is set, in connection with the activemode. The engine controller 28 obtains a fuel injection amount bylooking up a fuel injection characteristic map (not shown) with anengine revolution speed signal based on each engine output torquecharacteristic map, and then outputs a drive signal indicative of theobtained fuel injection amount to the fuel injection device 40. It ispossible to set an engine output torque characteristic line EL_(B)′having the isochronous control line Ld used in the second embodiment, inplace of the engine output torque characteristic line EL_(B) (this isalso applied to the fourth embodiment described later).

In the pump controller 33, pump absorbing torque characteristicscorresponding to the lifting mode and the active mode respectively aremapped and stored. Herein, in the third embodiment, a pump absorbingtorque characteristic line PL_(A), which undergoes a transition withequi-horse-power, is set in connection with the lifting mode. The pumpabsorbing torque characteristic line PL_(A) matches the engine outputtorque characteristic line EL_(A)′ at an output torque point Ma on theisochronous control line La. In the third embodiment, a pump absorbingtorque characteristic line PL_(B), which is a monotonically increasingfunction with the revolution speed of the engine serving as a variable,is set in connection with the active mode. This pump absorbing torquecharacteristic line PL_(B) matches the engine output torquecharacteristic line EL_(B) at an output torque point Mb at which theoutput of the engine 16 has a maximum. The pump controller 33 obtains aswash plate drive signal based on each pump absorbing torquecharacteristic map and outputs this swash plate drive signal to theswash plate drive unit 32.

In the third embodiment, if the operator turns ON the lifting modeselector switch 23 among the operation mode selector switches 24, theengine output torque characteristic line EL_(A)′ shown in FIG. 6 is set,while the pump absorbing torque characteristic line PL_(A) is set whichmatches the engine output toque characteristic line EL_(A)′ at theoutput torque point Ma on the isochronous control line La. On the otherhand, if the operator turns ON the active mode selector switch 21 amongthe operation mode selector switches 24, the engine output torquecharacteristic line EL_(B) shown in FIG. 6 is set, while the pumpabsorbing torque characteristic line PL_(B) is set which matches theengine output torque characteristic line EL_(B) at the output torquepoint Mb at which the output of the engine 16 has a maximum.

According to the third embodiment, in the lifting mode, the outputtorques in the middle and lower speed regions of the engine outputtorque characteristic line EL_(A)′ are slightly lower than the outputtorques in the middle and lower speed regions of the engine outputtorque characteristic line EL_(A) of the first embodiment. Therefore,the third embodiment has not only the same effect as the firstembodiment but also an advantage over the first embodiment in terms ofthe reduction of fuel consumption when the lifting mode is selected.

Next, a fourth embodiment of the invention will be described below. FIG.7 is a block diagram showing a schematic structure of anengine/hydraulic control system according to the fourth embodiment. FIG.8 is an engine output torque characteristic graph according to thefourth embodiment. In the fourth embodiment, parts that are identical orsimilar to those of the foregoing embodiments are once again indicatedwith the same reference numerals as in the foregoing embodiments.Although a detailed description of them is omitted herein, only partsinherent to the fourth embodiment will be chiefly explained below.

The engine controller 28 stores an engine output torque characteristicwhich is represented by the line EL_(A)″ shown in FIGS. 7, 8 and mappedas the engine output torque characteristic corresponding to the liftingmode. This engine output torque characteristic line EL_(A)″ has theisochronous control line La and is such that the output torques of themiddle and low speed regions are slightly lower than those of the engineoutput torque line EL_(A). The engine output torque characteristic lineEL_(A)″ further has a control line Le which leads to the isochronouscontrol line La. Herein, the control line Le is for allowing the outputof the engine to undergo a transition with a substantiallyequi-horse-power (the control line Le is hereinafter referred to as“equi-horse-power control line Le”). According to the engine outputtorque characteristic line EL_(A)″, the engine 16 is once drivenaccording to the isochronous control line La after the load starts toincrease from the unloaded condition. If the engine output torque valuewhich the load requires still increases after reaching a specified valueTs, the engine 16 is driven according to the equi-horse-power controlline Le.

The pump controller 33 stores the pump absorbing torque characteristicwhich is represented by the line PL_(A)′ in FIGS. 7, 8 and mapped as apump absorbing torque characteristic corresponding to the lifting mode.This pump absorbing torque characteristic line PL_(A)′ is amonotonically increasing function with the revolution speed of theengine serving as a variable and matches the engine output torquecharacteristic line EL_(A)″ at an output torque point Mc on theequi-horse-power control line Le.

In the fourth embodiment, if the operator turns ON the lifting modeselector switch 23 among the operation mode selector switches 24, theengine output torque characteristic line EL_(A)″ shown in FIG. 8 is set,while the pump absorbing torque characteristic line PL_(A)′ is set whichmatches the engine output toque characteristic line EL_(A)″ at theoutput torque point Mc on the equi-horse-power control line Le. On theother hand, if the operator turns ON the active mode selector switch 21among the operation mode selector switches 24, the engine output torquecharacteristic line ELB shown in FIG. 8 is set, while the pump absorbingtorque characteristic line PL_(B) is set which matches the engine outputtorque characteristic line EL_(B) at the output torque point Mb at whichthe output of the engine 16 has a maximum.

In the fourth embodiment, if the lifting mode is selected, the loadstarts to increase from the unloaded condition, so that the engine 16 isonce driven according to the isochronous control line La. If the engineoutput torque value which the load requires still increases afterreaching the specified value Ts, the engine 16 is driven according tothe equi-horse-power control line Le. Then, the actual revolution speedof the engine 16 converges on an engine revolution speed Nc whichcorresponds to the output torque point Mc (hereinafter referred to as“matching point Mc”) at which the engine output torque characteristicline EL_(A)″ intersects the pump absorbing torque characteristic linePL_(A)′. During this time, the engine output torque varies according tothe equi-horse-power characteristic of the engine 16 itself andtherefore increases, while the engine revolution speed is graduallyvarying in relation with increases in the load. When the output torqueof the engine 16 converges upon the matching point Mc, the output of theengine 16 is kept to be the value corresponding to the engine outputrequired at the matching point Mc, so that the engine 16 does not lapseinto an excess output.

According to the fourth embodiment, high loaded operation can be carriedout in a good condition without loosing fine controllability and fuelconsumption can be more effectively cut down when the lifting mode isselected, compared to the third embodiment.

Although the foregoing embodiments have been discussed in terms of acase where the invention is applied to a hydraulic excavator, theinvention is obviously applicable to construction machines, industrialvehicles, agricultural machines etc. other than hydraulic excavators.

1. A work machine comprising: hydraulic actuators activated by pressureoil from a hydraulic pump driven by an engine; an implement driven byactivation of the hydraulic actuators; an engine control device forcontrolling output of the engine in accordance with each of a pluralityof operation modes which are set according to contents of operations,the plurality of operation modes including a finely-controlled operationmode in which the implement is allowed to operate at ultraslow speed,and a revolution speed operation mode in which a revolution speed forthe engine is set to a value in a vicinity of a rated output revolutionspeed; and operation mode selecting means for selecting any one of theplurality of operation modes, wherein the engine control device isarranged such that when the finely-controlled operation mode has beenselected from the plurality of operation modes by the operation modeselecting means, the engine control device performs isochronous controlfor maintaining the revolution speed of the engine at a substantiallyconstant value irrespective of load fluctuations, and wherein the enginecontrol device is arranged such that when the revolution speed operationmode has been selected from the plurality of operation modes by theoperation mode selecting means, the engine control device performsregulation control for increasing or decreasing the revolution speed ofthe engine according to load fluctuations.
 2. The work machine accordingto claim 1, wherein the engine control device performs equi-horse-powercontrol subsequently to the isochronous control, if an output torquevalue of the engine which the load requires still increases afterreaching a specified value when the finely-controlled operation mode isselected.
 3. A work machine comprising: hydraulic actuators activated bypressure oil from a hydraulic pump driven by an engine; an implementdriven by activation of the hydraulic actuators; an engine controldevice for controlling output of the engine in accordance with each of aplurality of operation modes which are set according to contents ofoperations, the plurality of operation modes including afinely-controlled operation mode in which the implement is allowed tooperate at ultraslow speed, and a revolution speed operation mode inwhich a revolution speed for the engine is set to a value in a vicinityof a rated output revolution speed; and operation mode selecting meansfor selecting any one of the plurality of operation modes, wherein theengine control device is arranged such that when each of thefinely-controlled operation mode and the revolution speed operation modehas been selected from the plurality of operation modes by the operationmode selecting means, the engine control device performs isochronouscontrol for maintaining the revolution speed of the engine at asubstantially constant value irrespective of load fluctuations.
 4. Thework machine according to claim 3, wherein the engine control deviceperforms equi-horse-power control subsequently to the isochronouscontrol, if an output torque value of the engine which the load requiresstill increases after reaching a specified value when thefinely-controlled operation mode is selected.